Structured documents are documents which have nested structures. Documents written in Extensible Markup Language (XML) are structured documents. XML is quickly becoming the standard format for delivering information over the Internet because it allows the user to design a customized markup language for many classes of structure documents. For example, a business can easily model complex structures such as purchase orders in XML form and send them for further processing to its business partners. XML supports user-defined tabs for better description of nested document structures and associated semantics, and encourages the separation of document content from browser presentation.
As more and more businesses present and exchange data in XML documents, database management systems (DBMS) have been developed to store, query and retrieve these documents which are typically stored on direct access storage devices (DASD), such as magnetic or optical disk drives for semi-permanent. Some DBMSs, known as relational databases, store and query the documents utilizing relational techniques, while other DBMSs, known as native databases, store the documents in their native formats. XML documents are typically grouped into a collection of similar or related documents. Thus, for example, a group of purchase orders can form a collection.
Once a collection of documents is stored in the database, relational or native, it is potentially available to large numbers of users. Therefore, data security becomes a crucial concern. In particular, the DBMS must be able to control, i.e., deny or grant, access to the data by the user. In a conventional relational DBMS where the data is stored in rows and columns in tables, security is generally directed to the table level, i.e., access to a table is controlled. While this may be sufficient for relational data, it is inadequate for controlling access to a collection of XML documents because an XML document stored in the database contains information that is much more diverse than data stored in rows in tables.
Access control for XML documents is fine-grained, that is, access to each node in an XML document is controlled. The term “node” is used in the DOM-sense, which is a standard XML construct well known to those skilled in the art. In that construct, the XML document is represented by a plurality of nodes that form a hierarchical node tree. Each node of the XML document is identified by a path that defines a hierarchical relationship between the node and its parent node(s). Thus, fine-grained access control to the nodes of an XML document is referred to as path-level access control.
For example, if an administrator wanted to limit access to a “salary” node in all documents in a collection “all_employees,” the administrator would generate the following statement:    Deny read access on “/employee/salary” in collection “all_employees” to group non-managersThis statement would deny access to all salary nodes with path “/employee/salary” in all documents in collection “all_employees.” This type of statement is referred to as an access control rule. A set of access control rules directed to a collection of documents is referred to as an access control policy.
While it is possible to perform path-level access control evaluation by utilizing access control rules, such evaluation is relatively expensive because the DBMS must evaluate each access control rule to determine whether a user should be granted or denied access to data in a node. This process becomes prohibitive when the number of access control rules in a policy increases. Nevertheless, the alternative, i.e., coarse-grained access control or table level access control, is unacceptable.
Accordingly, a need exists for an improved method and system for providing path-level access control for structured documents stored in a database. The method and system should be integrated (or capable of being integrated) with an existing database system in order to use the existing resources of the database system. The present invention addresses such a need.